Until recently it was very easy to manipulate the price. If you sold off a thousand BTC (just to throw out a random amount) and then bought after the sledding stopped you could multiply your stash.

It's not as easy to do that now and will become much harder as time goes on. The amounts required to scare the market will become increasingly higher as confidence goes up and the possibility of a return decreases.

The single most importan parameter is the number of users. On top of that, we have speculation. The speculators considers the news, and different speculators make wildly wrong short time predictions in both directions based on it.

One thing that people are forgetting is how many new technologies, products, and services come out for Bitcoin. They make Bitcoin more attractive, draw in new users, and also accelerate the velocity of money. They are key drivers for Bitcoin.

Bitcoin is functioning in a chaotic free market economy and is dictated by that. If say there is Bad news in China or another country regarding Bitcoin there is usually a sell off. If the big whales decide to do a pump and dump there is a sell off. If there is good news regarding bitcoin like some new business taking it on or another bitcoin startup is successful then price goes up as well as the demand driving the price up more and more over time as more and more people want to own and buy bitcoins.